The high-temperature microscope is intended for observing the structure of a specimen which is heated to higher than room temperature and for investigating the structural changes in a specimen which is heated to high temperature or cooled to room temperature from high temperature.
Conventionally the specimen used to be held in a radiation heating furnace and was indirectly heated by radiation heat from a heating element attached internally around the furnace or was directly heated by passing electric current through it when it was a metal.
When the specimen is heated in such a conventional furnace for high temperature metallography, the maximum heat attainable is 1700.degree. C., because of the limit in the heat durability of the refractory materials used in the furnace. When a furnace is used, the heat capacity of the furnace is so large that the heat loss is heavy; and since the heat transfer is slow, the response is not quick.
When the specimen is directly heated by passing electric current through it, the specimen must be a thin strip conductor. The maximum temperature attainable is limitted by the specimen shape and the temperature distribution is not even.
In any of the conventional methods, only a simple, overall heating of the specimen is possible.
To prevent oxidation, the specimen is heated in a vacuum, and much gas is generated not only from the specimen but also from the refractory materials which is vaporized. Then, the specimen is contaminated with the vapor or it reacts with the vapor, thus impeding accurate observation. Further, the vapor deposits on any low-temperature part, notably on the observation window, thereby clouding the glass and inconveniencing observation and photography.
In the conventional method, observation and photography of the specimen are done utilizing a mixture of reflection from the beam which illuminates the specimen and heat radiation from the specimen exposed to high temperature. Thus for the following reasons, there is a limitation to the high temperature of the specimen under which it can be observed or photographed.
Namely, when the specimen temperature is high, heat radiates from the specimen, but the radiation beam contains visible rays as well as invisible ones. The visible rays mix with the reflection of the beam which illuminates the specimen and in consequence, the image of the specimen to be observed or photographed is flared and blurred with reduced contrast.
Since the brightness of radiation beam increases with a rise in the specimen temperature, the proportion of radiation beams in the visible rays grows and accordingly the influence of the interfering rays becomes that much greater.
Therefore, there is naturally a limit to the high temperature of the specimen which permits its observation and photography.
Meanwhile, it is common practice in measuring the specimen temperature to use a thermocouple directly attached to the specimen by, say, welding. However, such a measuring apparatus cannot accurately measure the temperature on the observed surface of the specimen.
The temperature distribution of the specimen as a whole is not always uniform and the temperature indicated by the thermocouple is the temperature of the specimen portion with which the thermocouple is in contact and not the temperature of the other specimen portion observed which is separated from the contact with the thermocouple. Thus the thermocouple is not an accurate device for measuring the temperature of the observed portion of the specimen.
Particularly when the specimen is thin with a low heat capacity, the heat transfer from the thermocouple to the specimen becomes too much to be neglected; when the specimen is heated or its temperature drops from high to normal, and the temperature distribution of the specimen becomes inordinately uneven with the result that the measured value of temperature is extremely inaccurate.
For these reasons a heating device has been demanded which can heat the specimen to higher temperature, is quick in response, is highly efficient, and heats only the specimen without affecting anything around it. The device should be one that can do a variety of heatings; not only simple, overall heating of the specimen, but also localized heating of it. The device should be one that will not cloud the observation window or least is not likely to cloud it. The device should be one which permits observing or photographing a clear image of the specimen without being influenced by heat radiation beam, and it should accurately measure the specimen temperature even when the specimen is hot.